The present invention relates to a printed circuit board and a method for its manufacture.
For information-technology and communication-technology devices, there exists an ever greater demand for signal-transmission paths which, having low susceptibility to failure and high data rates, permit data transmission that is reliable, fail-safe, and at the same time cost-effective. This development is essentially attributable to the dramatic increase in the capability of modem microelectronics for computer systems and their peripherals. In the field of mobile communication systemsxe2x80x94e.g., for motor vehiclesxe2x80x94there are also important demands such as savings in weight and ease of installation and service. This development is intensified by the introduction of interconnected systems for information and communication.
While optical signal transmission for longer distances has been introduced, its broad use in the application field of smaller, possibly mobile LANS (Local Area Networks), or even for signal transmission on the modular level, is prevented by the additional costs still to be borne at the present time for optical communications transmission engineering. However, because of the relatively small distances to be bridged (approximately 500 m maximum), the possibility offers itself of falling back on optical multimode waveguide systems for use in the short-wave spectral range (about 630-850 nm), since here it is possible to reduce costs because of the reduced geometrical tolerances and the less stringent demands on the attenuation deficiency of the materials. Inexpensive plastic fibers exist having sufficiently low attenuation, and suitable optoelectronic transmitter and receiver components are already commercially available or will soon be introduced into the marketplace. At the moment, various bus systems for broad-band signal transmission are standardized in the consumer electronics field, the real-time capability being of particular importance for high-quality transmission of audio-visual information. Optical data transmission via multimode fibers plays an important role here, whose importance will continue to grow in the future.
If various electronic devices, or even modules within a device, are to be operated in a system that is networked, for example, with plastic fibers, then further need arises for a cost-effective optical connection system. Passive optical components are necessary for the flexible configuration of networks of different topologies. Optical beam-shaping can be utilized to adapt the beam cross-sections, predefined by the optoelectronic components, to that of the multimode fibers, so that power losses can be kept to a minimum.
Since for the most part, electronic devices are constructed on the basis of printed circuit boards, the necessity results for implementing optical signal-transmission paths between individual modules of a printed circuit board or between different printed circuit boards, for implementing passive optical components such as power dividers, power combiners or star couplers, as well as for cost-effective methods for coupling optical fibers. Described in the following is a construction and a method for producing hybrid electrical/optical printed circuit boards which permit cost-effective implementation:
of optical multimode interconnect lines with passive structures;
of coupling points for optical fibers and for optoelectronic components for transmitting and receiving optical signals; and
of conventional electrical signaling links on a shared carrier.
In modem printed circuit board technology, complex, multilayer, flexible or rigid layer systems are used as carriers which are suitable for fitting with surface-mounted devices (SMD). The selection of different base materials makes it possible to implement electrical signaling links having defined impedance and controlled signal propagation times. Typical, well-controlled structure measurements (widths and clearances of printed circuit traces) lie at approximately 40 xcexcm. Epoxy-resin glass cloth (e.g., FR-4) is often used as substrate material for printed circuit boards. Multilayer printed circuit boards have been constructed from the most varied material combinations, e.g., from Teflon and FR-4 or Teflon and PMMI. The use of additional layers to compensate for different coefficients of thermal expansion is also known in principle.
Optical waveguide structures for single-mode and multimode operation have been implemented using many different methods in various material systems, among them also being plastic films. Among the numerous manufacturing methods, replicating methods are particularly cost-efficient if the necessary geometric structural fidelities lie in the range of a few micrometers, i.e., if multimode structures are intended to be used.
One possibility for producing waveguides is to pattern a core layer that has been applied on a substrate, using an embossing method in such a way that a pattern of depressions results corresponding to the desired waveguide arrangement, and to subsequently fill up these depressions with a further optically transparent material. This second material must have a higher refractive index compared to the substrate material, so that light can be conducted in the waveguide. An additional covering layer, having a lower refractive index compared to the waveguide core, is necessary so that the adjacent, non-transparent layers in the printed-board structure do not disturb the light propagation in the optical layer system. An example for a replicating method of producing optical multimode waveguide structures is described in the European Patent No. 0 480 618.
Another possibility for producing waveguides utilizes photolithographic processes. Usually a transparent substrate layer is first coated with a film of light-sensitive material. Chemical bonds in this material can be so altered by irradiation with short-wave light that either the component of the material exposed to light or the component not exposed to light becomes soluble by a solvent, and therefore can be removed in a further work step. For example, after the irradiation, it is thus possible to remove the material out of the provided waveguide cores, thus leaving behind the waveguide cores as a ribbed pattern on the substrate layer. This pattern is finally supplemented by a transparent covering layer to again obtain a level surface for the subsequent laminating steps. To permit optical waveguiding, the refractive index of the patterned waveguide cores must be greater than the indices of the adjacent layers.
The optical coupling of SMD-mounted optoelectronic components via reflecting facets to waveguide structures which have been implemented in additional polymer layers on the surface of printed circuit boards has been demonstrated (Thomsen, J. E., H. Levesque, E. Savov, F. Horwitz, B. L. Booth, J. E. Marchegiano, xe2x80x9cOptical Waveguide Circuit Board with a Surface-Mounted Optical Receiver Arrayxe2x80x9d, Optical Engineering, vol. 33, no. 3, 989 (1994)).
The present invention described here lies in the expansion of the layer system of conventional printed circuit boards by an inner-lying system composed of at least one transparent layer in which an optical waveguide pattern is defined, for example, using molding methods or by a photolithographic arrangement. After the entire layer stack has been laminated, a compact structure results, similar to conventional printed circuit boards, having additional optical patterns in the interior.
The implementation of optical signaling links in one or more additional transparent inner layers of a printed circuit board using largely conventional techniques of the printed-circuit-board industry goes beyond the related art. The same holds true for the optical coupling of optoelectronic components to such inner layers by way of beam-deflecting facets.
The optoelectronic components can be coupled to inner-lying, optical, multimode waveguides by reflecting microprisms which, together with the components to be coupled, are placed in such a way when assembling the printed circuit board that their reflecting surfaces lie in openings provided for them (for example, produced by bore holes).
The present invention is preferably used for multilayer printed circuit boards whose layer structure includes a system of optically transparent materials having optical multimode strip waveguides.